1. Field of the Invention
The present invention relates to an ion current detection apparatus for detecting the combustion condition of an internal combustion engine by detecting ionization, by way of an ion current, of combustion gas resulting from combustion in an internal combustion engine.
2. Description of Related Art
FIG. 10 is a circuit diagram of a conventional apparatus comprising an ion current detection apparatus 300 for the ignition apparatus 200 of an internal combustion engine. The ignition apparatus 200 comprises a motor vehicle battery or other electrical power supply 201, an ignition coil 202, ignition control circuit 203, and a spark plug 204. The ignition control circuit 203 comprises a switching circuit 210, a resistor 211, and a control circuit unit 212 for controlling the switching circuit 210.
The switching circuit 210 comprises npn power transistors 215 and 216 in a compound connection, zener diode 217, and resistors 218 and 219.
The ion current detection apparatus 300 comprises an ion current detection circuit unit 301 for detecting an ion current, a capacitor 302, and a zener diode 303.
Referring to the ignition apparatus 200, current is supplied from the electrical power supply 201 to one end of the primary coil La of the ignition coil 202; the other end of the primary coil La is grounded through the ignition control circuit 203. One end of the secondary coil Lb of the ignition coil 202 is grounded through the spark plug 204, and the other end is connected to the ion current detection apparatus 300, that is, to the cathode of the zener diode 303 and to one side of the capacitor 302. The anode of the zener diode 303 is grounded, and the other side of the capacitor 302 is connected to the ion current detection circuit unit 301. It should be noted that the anode of the zener diode 303 is shown grounded in FIG. 10, but can be alternatively connected to the ion current detection circuit unit 301.
The cathode of zener diode 217 is connected to the collector of power transistor 216, and the anode is connected to the base of power transistor 216, to protect power transistors 215 and 216 from counterelectromotive force from the primary coil La of the ignition coil 202. The junction between the resistor 211 and emitter of power transistor 215, and the grounded side of the resistor 211, are connected to the control circuit unit 212. A control signal from the engine control unit (not shown in the figure) is input to the control circuit unit 212 for controlling the ignition timing based on various engine operation information. The control circuit unit 212 controls the switching operation of the power transistors 215 and 216 based on the supplied control signal.
When the power transistors 215 and 216 are switched on by a control signal from the engine control unit (ECU below) in this configuration, a current of up to between ten and twenty amperes flows to the primary coil La of the ignition coil 202. A counterelectromotive force then occurs between the primary coil La and the power transistors 215 and 216 when the current supply from the primary coil La is suddenly cut off as a result of the power transistors 215 and 216 switching off in response to a control signal from the ECU after supplying current to the primary coil La for a specified time. The zener diode 217, however, normally limits the power supply between the collector and base of the power transistor 216 to approximately 300-400 V.
When a counterelectromotive force occurs at the primary coil La of the ignition coil 202, a voltage proportional to the winding ratio between the primary coil La and secondary coil Lb occurs at the secondary coil Lb. For example, because the number of windings in the secondary coil Lb is approximately 100 times the number of windings in the primary coil La, a voltage of approximately 30 kV occurs at the secondary coil Lb. The secondary coil Lb is connected such that a negative voltage occurs on the spark plug 204 side of the coil, and a positive voltage occurs on the side on which the capacitor 302 and zener diode 303 are connected. If the voltage stored by the capacitor 302 is less than or equal to the zener voltage of the zener diode 303 when the spark plug 204 sparks, a current of several ten milliamperes to a hundred and several ten milliamperes flows to the capacitor 302; if said stored voltage exceeds the zener voltage, the current flows from the cathode to the anode of zener diode 303.
As thus described, the counterelectromotive force of the primary coil La of the ignition coil 202 rapidly attenuates, the voltage at both ends of the secondary coil Lb also simultaneously drops rapidly, and the voltage at both ends of the secondary coil Lb drops ultimately to zero after ignition. The voltage stored in the capacitor 302 is then added to the potential of the secondary coil Lb, becomes approximately equal to the zener voltage of the zener diode 303 during the ignition operation, and a voltage equal to the zener voltage of the zener diode 303 is applied to the spark plug 204.
When a voltage comparable to the stored charge of the capacitor 302 is applied to the spark plug 204 inside a cylinder containing ionized combustion gases immediately after ignition, an ion current flows. Because capacitor 302 supplies this ion current, a current matching the ion current also flows to the ion current detection circuit unit 301 connected to the capacitor 302. This current is detected, and the signal contained in the ion current is processed.
The ion current is known to react to minute changes in the temperature and pressure inside the cylinder, and a device for detecting whether normal combustion is occurring by comparing the absolute value of this ion current has been disclosed in Japanese Patent Laid-Open Publication H7-217519 (1995-217519) filed by an inventor of the present invention. A circuit for extracting an oscillation wave component superimposed on this ion current as a means of detecting knocking caused by abnormal pressure inside the cylinder is also disclosed in Japanese Patent Laid-Open Publication H9-15101 (1997-15101), also filed by an inventor of the present invention.
With a conventional ion current detection apparatus, however, a voltage limiting element, such as a zener diode 303 for limiting the voltage of the capacitor 302 supplying the ion current, is required for each capacitor 302, and a significant power loss occurs due to the several ten milliampere to a hundred and several ten milliampere current and the approximately 100-400 V limit voltage flowing during ignition. The zener diode 303 or other voltage limiting element must be built with a heat radiation design sufficient to withstand such a power loss, thus contributing to increased cost.